Harris Worm Castings are the microbial-rich waste by-product that earthworms produce after eating nutrient-rich organic matter (e.g., leaves, plants, fruit). Often considered the "superfood of gardening", worm castings contain organic material beneficial for all your gardening needs. In addition, worm castings improve soil structure, water retention and drainage. They are extremely water absorbent, able to hold 2-3 times their weight in moisture. Added to soil or potting mixes, worm castings actually improve soil structure and can help increase plant yield and size. It can be used alone or with other gardening soils to enhance growth.